I used to breed and sell shrimp, for profit. I didn't sell much to individuals though. I produced thousands and sold them to fish and pet stores. If you calculate the cost of keeping your tanks going, it will take a long time to reach the point where you make a profit. Also, three tanks, even 10g tanks are not going to produce the quantities to be a reliable source of income. From what I understand, most people who sell here make enough to keep their aquatic addiction

going. Some may be making more, but I doubt that it would be enough to require filing taxes and getting a business license.
FYI, you don't need to spend large amounts of money on aquariums. I used large heavy duty plastic tubs for nursery/grow out tanks. If you, or anyone wants to go that route, here's how I set them up.
The idea is to keep the temporary nursery tanks sparse, economical, and consistently cycled. To do this I introduced mystery snails to the tubs, put a few handfuls of ceramic ring/tube filter media that had been in healthy cycled tanks, a few sponge filters and kept them in a room with consistent temps in the high 70's. When I was ready, I just took a berried female and floated her in a breeder cage. This way I could get multiple broods in one tank, of the same approximate age. I raised the shrimp (Macrobrachium shrimp) until they were over a 1/2". Then I sold them off in bunches of ~500 or so. Since the tanks had no substrate, I could quickly scoop out the filter media then vacuum up the young shrimp with a siphon. I had four tubs going at all times, and another one cycled, if needed. I designed the system so that I could ramp it up, if someone wanted more. I had 12 tanks with breeding adults, one male to two females. I found that with that configuration I constantly had berried females. When shrimp were hatched in a breeding tank, I moved them, as I could, to one of two larger communal tanks. I sold shrimp from those online to other hobbyists.